1. Field of the Invention
The present invention relates generally to radio positioning systems and in particular to the augmentation of GPS using time offset code modulation.
2. Problems in the Art
Currently, there are at least 24 satellites orbiting earth dedicated to providing service for the Global Positioning System (GPS). GPS is run and managed by the U.S. Department of Defense. GPS satellites transmit a pseudo random noise code signal to receivers on the earth or in its atmosphere. Such receivers are designed to search for a code that matches a code stored in an almanac which is carried by the receiver. The almanac tells the receiver where the satellite, or the source of that particular code, is located in space. Searching is typically done only for a specified amount of time. Upon matching the sent code to the stored code prior to the end of the search time, the receiver obtains a lock on the signal. Through mathematics the receiver may use this signal along with measurements from three or more other satellites to determine. the distance the receiver is from the satellite. Such information may then be used for various purposes, such as computing the location of the receiver on a map, the distance the receiver is from the ground or any other set reference point, or determining time to a high degree of accuracy.
Pseudo satellites, commonly referred to as pseudolites, are essentially non-orbiting transmitters which transmit a signal identical or similar to the pseudo random noise code sent out by GPS satellites.
Pseudolites are used for a wide variety of purposes, including enhancing GPS signals, providing GPS like positioning within a line of sight barrier, and various other ranging, surveying and positioning purposes. While pseudolites work well for those situations, they can cause significant errors in conventional GPS applications.
Because of the reuse of GPS signals, Pseudolite transmissions are often indistinguishable from actual satellite transmissions. As most GPS applications require the GPS receiver to base its calculations on the position of the GPS satellite, when the same signal is received from a different location without the receiver""s knowledge, the calculations of the receiver will likely have large errors associated with them if the receiver is able to achieve a lock on a signal at all.
To overcome these errors, it is desirable to be able to distinguish a pseudolite from a real orbiting satellite.
As GPS satellites are in orbit around the earth, hardware modifications would be very costly and endanger human lives.
It is therefore desirable to be able to distinguish between a pseudolite and a satellite without hardware modifications. There is therefore a need for a positioning system which avoids these and other problems.
A general feature of the present invention is the provision of a positioning system which overcomes the problems and deficiencies found in the prior art.
A further feature of the present invention is the provision of a positioning system which is capable of distinguishing between satellite and pseudolite transmissions.
A further feature of the present invention is the provision of a positioning system which is capable of distinguishing between satellite and pseudolite transmissions without the need for hardware modifications.
Another feature of the present invention is the provision of a positioning system which allows the re-use of currently available pseudo random noise codes for many differing applications, all of which may operate simultaneously using the same pseudo random noise code.
These, as well as other features and advantages of the present invention, will become more apparent from the following specification and claims.
The present invention generally comprises a radio positioning system, and in particular a positioning system which is modified to include the augmentation of GPS using time offset code modulation. The present invention uses a transmitter to send a pseudo random noise code signal to a receiver. The sent signal includes a time offset. The receiver is programmed to account for the time offset and receive the pseudo random noise code signal.